An oil pressure control device is widely known in the art. In the oil pressure control device, a spool is movably accommodated in a sleeve, so that the spool is reciprocated inside of the sleeve. The sleeve has an inlet port into which working oil is inputted, an outlet port from which the working oil is outputted and a discharge port from which a part of the working oil flowing from the inlet port to the outlet port is discharged. In the oil pressure control device of this kind, an electromagnetic thrust force is generated by a solenoid in accordance with a command value of electric current. The electromagnetic thrust force and an elastic restoring force of an elastic member (a spring) are applied to the spool in an axial direction of the spool. The electromagnetic thrust force and the elastic restoring force are opposed to each other in the axial direction, so that the spool is reciprocated in the sleeve. An amount of the working oil discharged from the discharge port is adjusted in order to control an output oil pressure of the working oil outputted from the outlet port.
For example, Japanese Patent Publication No. 2013-24406 discloses an oil pressure control valve, more exactly, a method for setting a characteristic of an output oil pressure with respect to a command value (equal to electric power supplied to the oil pressure control valve), that is, a command-response characteristic. More exactly, a command-response characteristic at a reference oil temperature is actually measured as a reference characteristic. And the command-response characteristics for each oil temperature are set by use of the reference characteristic.
In the oil pressure control valve, an inlet port, an outlet port and a discharge port, which are formed in a cylindrical wall of a sleeve, are arranged in this order. A spool has several lands arranged in an axial direction thereof, so that each of the lands of the spool is in a sliding contact with an inner peripheral surface of the sleeve, to thereby form interfacial surfaces between an outer peripheral surface of each land and the corresponding inner peripheral surface of the sleeve. In each of the interfacial surfaces, the outer peripheral surface of the spool and the inner peripheral surface of the sleeve are overlapped with each other in a radial direction of the sleeve.
An axial length of an overlapping area in the interfacial surface and a clearance width of the overlapping area in the radial direction have an influence for setting the command-response characteristic. It is difficult to get access from an outside to the interfacial surfaces in the sleeve. However, the above prior art (No. 2018-24406) takes the above influence into consideration. As a result, it is not necessary in the above prior art to actually measure the axial length and the clearance width of the overlapping area, when setting the command-response characteristic for each oil temperature. And thereby, in the above prior art, it is possible to increase productivity of the oil pressure control valves.
However, the inventors of the present disclosure found out, through their researches for setting the command-response characteristic, that the command-response characteristic varies even in a case both of the axial length of the overlapping area and the clearance width in the radial direction are not changed. In addition, the inventors found out that the command-response characteristic varies as a result that an electromagnetic thrust force and an elastic restoring force are inevitably changed because of their tolerances. Accordingly, it was found out that accuracy for setting the command-response characteristic is decreased.